The present invention refers to a method and a device for determining a powder quantity or change in powder quantity in a container. The invention can in particular be applied to the field of powder coating to determine and control a powder mass flow which is supplied to a coating device and which is sprayed by same.
To supply powder coating devices, e.g. in electrostatic coating apparatus, with coating powder, the powder is fluidized, i.e. the powder is interspersed with a gas, preferably air, to form a powder-air mixture, and this powder-air mixture is supplied to the coating apparatus via supply lines. It may become necessary to determine the actually transported and supplied powder quantity per time unit, i.e. to determine the powder-mass flow (in g/min), during the operation of the coating apparatus, for instance to adjust the powder mass flow to a desired value or to maintain an adjusted value by means of a control.
Means for measuring a powder mass flow are known from the prior art which serve for determining the powder quantity discharged by a coating apparatus. U.S. Pat. No. 4,941,778 describes a method and a means for measuring and controlling the powder quantity which is supplied per time unit to a spray coating means in a powder coating apparatus. In the method described, the density (c) of the powder is measured in the powder-gas flow by a beam measuring means, the gas quantity per time unit (b) in the powder-gas flow is determined and the powder quantity per time unit (c) is determined by the equation c=bxc2x7a.
A further example for the measurement of a powder mass flow that is supplied to a coating apparatus is described in U.S. Pat. No. 5,864,739. The device described in this specification measures two parameters, namely the speed V of the powder-air mixture in the supply line in m/sec and the powder mass per volume unit Mv in the supply line in g/cm3. The powder mass flow and therefore the powder quantity which is supplied to the coating apparatus and which is sprayed by same can be calculated from the product of powder density and speed of the powder-air flow. The measurement of the speed is made by means of measuring electrodes that are spaced from each other and detect charge fluctuations at the supply line generated by the supplied powder-gas mixture. The measurement of the powder mass per volume unit is made by means of a resonator, which determines a change of the dielectricity constant and/or of the absorption in a resonance volume of the supply line as a shift of the resonance frequency.
A different type of measurement of the sprayed powder quantity is based on the determination of the change in weight of a powder reservoir. The weight of the powder reservoir is determined by means of scales, and while the powder is supplied from the powder reservoir to the coating apparatus and is sprayed by same, the weight of the powder container is continuously measured to determine the powder quantity over time. Such a measurement of the powder ejection at powder spray apparatus is known from DE 43 44 701 A1.
The known measuring methods require measuring means, such as scales or a resonator and measuring electrodes, which are relatively expensive.
DE 30 39 210 C3 describes a device for the indication of a bulk density and filling level of grained catalysts in a fluidized bed reactor, wherein a plurality of measuring tubes of pressure difference measuring devices are arranged at the reactor. By means of a first pressure differential measuring device the bulk density is calculated and indicated and by means of a second pressure differential measuring device and the previously detected bulk density the filling level is calculated and indicated.
Thus, it is the object of the invention to provide a device and a method of measuring a powder quantity which provide reliable results at the smallest possible expenditure.
The method according to the invention is suitable for determining a powder quantity in a container and for determining a change in powder quantity, i.e. the powder quantity that is supplied from or to the container. The powder is fluidized in the container, the pressure of the fluidized powder in the container is measured to determine the pressure of the fluidized powder or the change of the pressure between two measurements, wherein the powder quantity is calculated from the pressure or the change in pressure which is located in the container, or which was supplied from the container between two measurements.
The method according to the invention enables by a simple pressure measurement to determine the powder quantity which is supplied per time unit from a container and which is sprayed by a coating apparatus to thereby determine the supplied powder mass flow. Scales or other expensive measuring means are not necessary for this purpose.
The invention utilizes the fact that a so-called fluidization pressure builds up in the container above the fluidization means due to the fluidized powder. Since fluidized powder basically has the characteristics of a liquid, the physical properties of liquids also partially apply. As in a liquid, a certain pressure prevails in the fluidized powder which is proportional to the depth; this fluidization pressure only depends on the type of powder and the powder quantity provided in the container. If the powder quantity or the depth of the fluidized powder rises or falls, the pressure in the container also rises or falls proportionally thereto. This effect can be utilized for determining the filling quantity or for determining the change of filling quantity in a powder container.
Thus, the powder quantity in a powder reservoir container can be calculated by the method according to the invention in absolute figures or the change of the powder quantity in the powder container can be calculated on the basis of pressure conditions within the container. Furthermore, the method according to the invention when being used accordingly is also suitable for determining the height or change in height of the fluidized powder in the container.
The measurement of the powder quantity in the container on the basis of the fluidization pressure can be calibrated in a simple manner in that a known powder quantity is given into the container and the increase in pressure that results from the addition of the known powder quantity is measured. From the relation of known powder quantity and pressure increase at a subsequent measurement of the pressure, the powder quantity or powder quantity change in the container can be derived from the measured pressure.
The invention also provides a device for determining a powder quantity in a container, which comprises a fluidization means in the container, a supply means for supplying powder from the container, a pressure measuring means for measuring the pressure of the fluidized powder in the container and a calculation means for calculating a powder quantity in accordance with the pressure of the fluidized powder in the container. This device can be used for determining the powder quantity or the change of powder quantity in the container.